1. Field of the Invention
This present invention relates to offset printing presses, and in particular to the water systems and to the control of the fountain rollers in these presses.
2. Description of the Prior Art
In offset printing presses, water is applied to a printing plate before the ink is applied to cause the ink to adhere only to the image portion of the plate as is well known in the offset printing art. If too much water is applied to the printing plate, the ink washes out, and if not enough water is applied, too much ink adheres to the printing plate and scumming or streaking appears on the printed product.
Water is usually applied to the printing plate through a series of rollers. A first, fountain roller is located in a tray of water and rotates to pick up the water. A ductor or metering roller contacts the fountain roller to transfer the water thereon to a transfer roller. The transfer roller is in contact with an oscillator roller which oscillates longitudinally to even out the water. The water is then transferred to a water form roller which is in contact with the printing plate on a printing cylinder to evenly coat the printing plate with the desired amount of water.
The key to the amount of water ultimately applied to the printing plate is the speed or amount of rotation of the fountain roller before the water is transferred therefrom through the ductor or metering roller to the transfer roller. In the past, the rotation of the fountain roller has been controlled by a ratchet mechanism whereby a toothed wheel or gear is mounted on the shaft of the fountain roller, and a reciprocating pawl device engages a selected number of teeth on the wheel to rotate the fountain roller an incremental amount representing the amount of water desired to be picked up by the fountain roller and transferred through the ductor or metering roller to the transfer roller.
A difficulty with the prior art apparatus, however, is that it is not possible to obtain a fine enough adjustment of the movement of the fountain roller using a ratchet and pawl mechanism. If the teeth are made too small allowing for finer adjustment, the teeth are prone to excessive wear or breakage and premature failure of the apparatus.
Attempts have been made to replace the ratchet and pawl mechanism in the prior art with other devices for controlling the rotation of the fountain roller. One example of this is the use of an electric drive, such as a stepping motor to drive the fountain roller. Other approaches have used various machine elements involving cams and linkages. A difficulty with the prior art devices however, is that the printing presses or color heads have to be custom designed or made especially to incorporate these different types of mechanisms. It has generally not been possible to retrofit an existing printing press or color head with a different type of fountain roller drive due to the complexity of these machines and the limitations caused by lack of space and the presence of other equipment.